N-type doping of an electron-transport material by controlled gas-phase incorporation of cobaltocene

N-doping of an electron-transport material, a tris(thieno)hexaazatriphenylene derivative (1), with the strongly reducing molecule bis(cyclopentadienyl)-cobalt(II) (cobaltocene, CoCp₂), is investigated using ultra-violet, X-ray, and inverse photoemission spectroscopies, and current-voltage measurements. Condensed CoCp₂ films show a 4 eV ionization energy, which is unusually low for vacuum-deposited molecular material and suggests that cobaltocene is promising as a molecular n-dopant. Efficient n-doping of 1 by CoCp₂ is confirmed by a 0.56 eV shift of the Fermi level toward the unoccupied states of the host, and by a three orders of magnitude current increase in devices where compound 1 is interfacially doped with cobaltocene.